In recent years, mechanistic chromatography modeling has become a staple for efficient bioprocess development. As most chromatography processes use a change in salt concentration or pH to elute protein from the chromatography column, accurate description of pH values and ionic strengths in buffers and their mixtures is important for modeling these processes. Many adsorption models include terms to account for changes in pH and ionic strength, so a good description of these is a prerequisite for a highly descriptive model.<br/>We present a method to calculate with high accuracy the buffer ion compositions and pH values in an overall simulation of the chromatography process. The method uses the Henderson–Hasselbalch equation in combination with the Davies equation for calculating ion activities depending on the ionic strength and composition of the running buffer. To account for temperature changes, the method also includes a linear dependency of pKa values on temperature. The resulting pH values and ionic strengths serve as input parameters for the calculation of adsorption equilibria within the simulation.<br/>This method improves the accuracy of descriptions of pH profiles and ionic strengths during chromatographic runs in which one or both conditions change, especially gradients. Models for these processes become more accurate, as nonlinearities in pH do not need to be “picked up” by unrelated parameters. Application across a wider pH range is also possible, as the model relies on a mechanistic description of pH rather than a linear extrapolation.

Mechanistic description of pH and ionic strength values in chromatography simulations

ACS Spring 2022

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